An avenue to understand how biological systems function at the molecular level is to probe biomolecules individually and observe how they interact with each other directly in vivo. Laser-induced fluorescence has been a technique widely adopted for this purpose, thanks to its ultrahigh sensitivity and capabilities of performing multiple probe detection.
In applying this technique to imaging and tracking a single molecule or particle in a biological cell, the progress is often hampered by the presence of ubiquitous endogenous components, such as flavins, nicotinamide adenine dinucleotides, collagens, and porphyrins that produce high fluorescence background signals. These biomolecules typically absorb light at wavelengths in the range of 300-500 nm and fluoresce at 400-550 nm. To avoid such interference, some biological fluorescent probes absorb light at a wavelength longer than 500 nm and emit light at a wavelength longer than about 600 nm. At these wavelengths, the emission has a long penetration depth through cells and tissues. Organic dyes and fluorescent proteins are frequently used to meet such needs. However, the detrimental photophysical properties of these molecules, such as photobleaching and blinking, can restrict their applications for long-term in vitro or in vivo observations.
Fluorescent semiconductor nanocrystals have a number of advantageous photophysical properties such as high photobleaching thresholds, broad excitation but narrow emission spectra, and allowing multicolor labeling and detection. Yet, most of them are toxic. Consequently, prior to use, their surfaces often have to be modified to reduce cytotoxicity and human toxicity. However, surface modification can change the photophysical properties of semiconductor nanocrystals, thus limiting the scope of their biological application.
There is a need to find an alternative labeling material which has both outstanding photophysical properties and are low- or non-toxic.